Equipment used in space applications, for example solar panels, antenna reflectors, solar sails, etc. as well as the satellites themselves have to be subjected to various simulated conditions for testing their suitability in various mechanical environments and in order to determine with certainty their physical characteristics. In particular, such pieces of equipment are subjected to sinusoidal or random vibrations. Their moments of inertia, centers of gravity and dynamic balancing are all determined with the aid of tests in motion.
Hitherto these tests have been effected either under vacuum or, in the great majority of cases, under ambient conditions.
Working under vacuum imposes conditions somewhat similar to those affecting the satellite and its various pieces of equipment in space. The test is carried out under a vacuum bell in which the test means and the equipment to be tested have been placed. Electric motors for moving the equipment have to be lubricated regularly. Under vacuum the lubricant vaporizes and it is thus not effective. This difficulty accordingly leads to the use of special and accordingly very costly apparatus for tests under vacuum. Furthermore sealed connections have to be made between the measuring gauges, located inside the bell, and the recording apparatus, located outside the bell. Finally the use of a rigid structure like the vacuum bell involves very high expense. To carry out tests under vacuum is thus complex, inflexible and costly.
This is why the majority of simulations are made under ambient conditions. Ambient conditions (in air at atmospheric pressure) are clearly different from conditions genuinely close to the vacuum of space. In particular the resistance of the ambient medium on the moving surfaces of the test equipment is very great and increases with the area of the equipment and with the square of the speed of movement to which it is subjected. Since this aerodynamic resistance is very difficult to model mathematically because of the complexity of the surfaces of the test equipment, it is not possible to correct the results of tests in such a manner that it no longer needs to be taken into account. In particular, determination of the moment of inertia or the center of gravity of a structure with a large surface area is effected by measuring characteristic parameters under an oscillatory movement which is very much perturbed by the aerodynamic resistance. This implies an error in the value of the moment of inertia or in the position of the center of gravity, involving an error during the dynamic balancing of the equipment and accordingly during the dynamic balancing of the satellite itself.
Furthermore the momentum needed to displace a certain volume of gas is directly proportional to the density of this volume of gas. For this reason, for given motor power, the greater the density of the ambient medium, the smaller the momentum obtained relative to the momentum needed. It is then not possible to obtain the desired movements.
Finally, during vibration tests, for example, with a view to determining the characteristic modal masses and frequencies of the structure, the density of the ambient medium involves modifications in the values of these modal masses and frequencies in the ratio of calculated values. In particular, the modal masses in ambient air are higher than in a vacuum. which leads to additional stresses on the tested equipment (since it has been dimensioned to operate under a vacuum), which can lead to the structure breaking.